The present invention relates to a high efficiency coding method for grayscale or color images.
Heretofore there have been proposed a variety of coding systems for still color images (including graded images) and, in particular, two international standard systems called a JPEG system and a JBIG system, developed jointly by CCITT SG VIII and IEC/JTC1/SC29, have lately attracted considerable attention.
The JPEG (Joint Photographic Experts Group) system, proposed as a still color image coding system, is not an information preserving type but a system which permits high efficiency coding of grayscale or color images.
The JPEG system adopts a discrete cosine transform (DCT) system, one of orthogonal transform systems which are highly efficient for still color images, in general. The DCT system is one that divides an image into blocks, conducts a discrete cosine transformation for each block and encodes the resulting output coefficient. The output coefficient is quantized independently for each frequency component and a particular number of quantizing bits is assigned for each transformation coefficient so as to provide for an optimum coding efficiency. Since the still color image contains a large amount of information, it is not practical to employ a coder free from information loss for coding the still color image, unlike in the case of coding binary images. The coding of the still color image calls for quantization at a suitable level and a coarse quantization in the high frequency region, in general, will provide a high degree of coding efficiency.
On the other hand, the JBIG (Joint Bi-level Image Expert Group) system, proposed as an international standard system for the coding of bi-level images, is an information preservation type coding method which permits high efficiency coding of images basically represented in a bi-level form, with no drop-out of information.
It is assumed that images for transmission by facsimile are multi-color documents mostly with monochrome letters and color images or color and monochrome letters and color images intermingled therein, rather than documents with color images alone. However, a coding system which employs the DCT technique, such as the JPEG system, has a shortcoming that the picture quality of a letter area is seriously deteriorated in the case of a document with letters and images intermingled therein. In view of the above, there have been proposed several methods which obviate the above-mentioned defect of the prior art and permits high efficiency coding of a document with binary letters and grayscale images intermingled therein or a binary color document.
As mentioned above, in the case of applying the conventional discrete cosine transform (DCT) coding method to the color image, the image quality is impaired by the generation of noise around edges of letters contained in the image, for the reason given below. In the DCT process edges of letters, which undergo sudden changes in their pixel values, are also quantized, and since the pixel values of such letter edges greatly differ from the adjoining pixel values, the image inevitably contains many high frequency components. Hence, efficient quantization of such an image is equivalent to its rough quantization, and as a result, noise around letters cannot be ignored.
Moreover, in the case of applying the above-noted prior art method to the coding of an image in which letter edge areas have been blunted or emphasized by scanning, the number of coding bits increases when the letter edge areas are very blunt, because image properties substantially differ with images.